Question

Among the following which one contains valence electrons in the ground state of the magnesium Mg atom ?
a.) 1s
b.) 2s
c.) 3s
d.) 3p
e.) 3d

Answer 1

Hint: The Magnesium and Sodium belong to the same period. These both metals have valence electrons in the same shell. Thus, the valence electrons of Mg will be in the same shell as that of Sodium. The K shell of Mg has 2 electrons in it, L shell has 8 electrons and the M shell also has 2 electrons.

Complete answer :
First, let us know some points about Magnesium.
Magnesium is an alkaline earth metal. It has atomic number 12. It is a shiny grey solid that occurs naturally in combination with the other elements. The free metal is not available naturally but can be synthesized artificially.
Another term important in question is valence electrons. Now, let us see what these are.
The outermost electrons of an atom that are not so tightly bound by the nucleus and can be donated whenever required are called valence electrons. The valence electrons are shared among the atoms during chemical bond formation.
The ground state refers to the natural state. When any atom acquires energy, it gets excited and the electrons jump out of it to higher states. The ground state is the lowest possible state.
If we talk about the ground state electronic configuration of the Mg atom. Then, we know that it has atomic number 12. So, there will be 12 electrons in an atom of Mg. The filling will come out to be as -
$1{s^2}2{s^2}2{p^6}3{s^2}$
So, the valence electrons i.e. the outermost electrons are in 3s orbital of the Mg atom.

Thus, the option c.) is the correct answer.

Note :
The filling of electrons takes place according to Aufbau's principle. It states that in the ground state of an atom, the electrons fill atomic orbitals of the lowest available energy levels before occupying the higher energy levels. The filling takes place in order 1s, 2s, 2p, 3s, 3p, 4s and 3d etc.